Color printing by lithography goes back to the early years of the nineteenth century. Multiple stones were used, one for each ink printed, and the print went through the press as many times as there were stones. The problem was then, as it is today, to keep these individual images in proper register. Considerable skill was required on the part of the printer to make sure that each color would be in the correct position and that the overlying colors would merge correctly. The process left much to be desired and up to the middle of the century the highest quality works were colored by hand. High costs severely limited availability of color illustration. The development of a number of photomechanical processes near the turn of the nineteenth century reduced the level of skill needed and greatly expanded the use of printed illustration. Processes such as photolithography, photogravure, photoengraving, and others, allowed the image to be photographically transferred from an original photograph or drawing to a printing plate. These processes initially were useful only for black and white illustration. The later invention of the trichromic halftone process was a major breakthrough in printing color images. Improved photographic films with wide spectral sensitivity allowed the use of individual camera filters which removed all colors except the red, green and blue spectral region of the original. This allowed the preparation of a series of halftone plates that printed the images in complementary cyan, magenta and yellow inks. So-called process printing, using subtractive primary Cyan, Magenta, and Yellow inks, along with blacK ink (CMYK) is now in wide use.
While equal amounts of superposed transparent cyan, magenta, and yellow inks would nominally produce black, the result is more often a muddy brown. The separate black ink is used to overcome the spectral impurities of the three subtractive primaries. With standard process printing, four image separations are needed, one for each ink color. A separation is a monotone (gray scale) image that indicates how much of a given color ink needs to be printed at a given location. For higher quality printing where a wider color gamut might be needed, other processes such as Hexachrome® can be used. Hexachrome is a registered trademark of Pantone, Inc., Carlstadt, N.J. This process adds orange and green to the basic CMYK colors. A black separation is also normally used to provide a wider range of image luminance and render better neutral grays and shadow density.
Common to all color reproduction processes is the need to hold tight register between the various separations during printing. For analog devices using printing plates on a press this is a major problem. Analog processes include those known as gravure, flexography, lithography, screen, and letterpress. In particular, the ability to hold register in the machine direction is generally worse than holding side-to-side register. Even a displacement as small as 0.085 mm (0.003 inch) can produce noticeable unpleasing artifacts that result in an unsatisfactory printed image. This small displacement translates into a single row of halftone dots at a 150 lines per inch screen ruling. The amount of image degradation relates directly to the amount the plates are out of register. Additionally, although to a lesser degree, maintaining exact register for various digital printing devices, such as ink jet or laser jet printers or short to medium run digital presses, is also needed.
With conventional color separation technology, the resulting image in each of the separations carries both object color and object detail information. This is extremely important to keep in mind. This system is entirely satisfactory for printing when all printing plates can be held in exact register. However, the method suffers badly when any of the plates is out of register. An out of register image can render multiple images of a given object, introduce unwanted edges, unwanted colors, apparent poor image resolution, and other unpleasant artifacts. The printed image gives the viewer an immediate sense that something is wrong.
One widely implemented technique used to minimize the problem of misregistration is called “image trapping”. An oversimplified explanation of this might be the situation where a well defined object is seen against a contrasting uniform background. If the area occupied by the object is made a bit larger than the space it would normally occupy (image spreading), the danger of a white edge appearing is reduced if the separations are somewhat misregistered. Alternatively, the background space that the image would occupy can be made somewhat smaller (image choking). Another alternative is to use a heavy border that overlaps the image. In all cases there is overlap of the image border onto the background area. U.S. Pat. No. 6,236,754 describes one somewhat different technique to mask misregistration at object borders.
The effectiveness of various trapping methods is limited since choking and spreading strategies deal primarily with borders between the boundary of a given object and its adjacent background. Details of edges within a given object are not addressed nor are complex images where there is poor separation between object and background.
The conventional solution to holding register of multiple overprinted images is to use complex sensors and feedback control to the printing hardware. This approach is expensive and is often impractical in many applications such as newspapers and corrugated packaging. In the past, quality requirements for these uses have been somewhat less stringent. Competition is now pressing even the low-end color printing applications to improve their quality.